Center of Gravity

Discussion in 'Stability' started by Bluwaterbrew, Oct 31, 2018.

  1. Bluwaterbrew
    Joined: Mar 2009
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    Location: North Carolina

    Bluwaterbrew Junior Member

    I am working on designing a 17' boat for my dad and I to build. It will be powered by a 4o or so horsepower tiller engine and used in the local rivers.

    I have a preliminary hull form drawn and want to make sure it will float/run properly before we start building it. I did some of the Westlawn yacht design light program several years ago and still have the books.

    My first step will be to establish a LWL and calculate the displacement. Then I will find the center of buoyancy. I am fine up to this point. When it comes to finding the center of gravity I am getting a little confused. West lawn tells you how to calculate the LCG and VCG based on the loads of various items in the hull (engine, gas tank, batteries, etc) however I want to find the LCG and VCG of the bare hull. How is this accomplished?

    I have drawn several hulls in CAD for fun but this is the first one that I actually plan on building so I want to make sure I do all of the necessary calculations. Thanks for any help!
     
  2. Bluwaterbrew
    Joined: Mar 2009
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    Bluwaterbrew Junior Member

    These are the most recent screen shots that I have of the hull but I have tweaked it a little.
     

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  3. Sharat
    Joined: Oct 2018
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    Sharat New Member

    If you want to find the weight of the bare hull,
    Hull Surface Area x Thickness of Laminate x Desnsity of Laminate.

    If you want to find Centroid of Bare hull,
    Find Centroid of the surface Area.

    I am willing to help, pls email your 3d model at jsharatchand@gmail.com
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    What is the material to be used for the hull ?
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    This is the correct way to perform the calculation but, to do it well, you must, first, define the hull/deck structure, calculate the scantlings and the laminates schedule.
     
  6. Sharat
    Joined: Oct 2018
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    Location: India

    Sharat New Member

    Dear Bluewaterbrew,

    I will give you guide of what to do..

    1. Finalise your material of construction, Is it Steel/Aluminum/FRP. I will assume it as FRP and go ahead.
    2. Finalise your type of construction single skin/sandwich. See which is available/easy for construction.
    3. Finalise your speed acheived from your OBM engine. do some powering calculations, I don't think it shall come under HSC, engine power is very low.
    4. Finalise your rules, I see you want to operate in rivers, it clearly indicates Inland operation. opt for inland rules of your nearest class society(ABS).
    5. Finalise your scantlings, I see you already made the model, assuming you have set of hydrostatic data, General Arrangement etc. proceed with scantling calculations.
    6. Finalise your Weight from Scantlings, Use a 3d Model tool for structural modelling, it shall give you structure weight estimation data from inside the software itself. for your info this weight and cg estimation will be exact.
    7. Finalise your additonal weights apart from bare hull like obm,batteries, navigation console, mast, railings, cleats, hatches, doors, windows etc.
    8. Finalise the stability, with the new total weight estimate(barehull +additional) check if stability criteria are meeting as per Class/Flag requirement.
    9. Finalise your final Draft is having sufficient freeboard, and final draft less than equal to scantling draft.

    thats it

    Thanks
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    All a bit unnecessary, I think, in a small boat like this, with no superstructure, the stability problems are likely to come from non-fixed weights, aka people ! also, high winds probably represent a greater risk. And the hollow flare forward in this boat represents a shaping complication that isn't really needed, imo.
     
  8. messabout
    Joined: Jan 2006
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    messabout Senior Member

    I second the comments that Mr, E made above.

    Tell us the general dimensions of the boat and if possible show what the sections look like, especially the mid section and section at the transom.

    What material will you be using?
     
  9. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Mr Efficiency is spot on, your proposed boat is in no danger of being unstable by design, just look around your local lake or river for similarly dimensioned craft, there will be many.
    I’d also say that the bow flare is excessive for the intended use, and adds complexity to the build.
     
  10. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Not sure why you want the CG of the bare hull but the easiest, most accurate way is to build a model out of cardboard.
    Then balance it on a dowel, fore and aft, install a small cross bar, tie a thread to it and hang to check accuracy.
    You can check the actual hull once complete the same way.
    You'll be within inches.
     
  11. Bluwaterbrew
    Joined: Mar 2009
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    Location: North Carolina

    Bluwaterbrew Junior Member

    Maybe I am over thinking this but I am trying to make sure that I have my hull trimmed correctly in relationship to the DWL. I have estimated the displacement to be 1000lbs and have calculated the LCB and the VCB based on how I have the hull trimmed in relation to the DWL.

    As I understand it the boat is trimmed properly when the CB and the CG line up vertically.

    I just want to make sure all of my calculations are accurate and that the hull is drawn correctly in relationship to the DWL.

    Hopefully that all makes sense. Also let me say I am not s professional. This is my first design and I am learning as I go. The hull will be cold molded. The flare is purely for aesthetics.
     
  12. Bluwaterbrew
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    Bluwaterbrew Junior Member

    Per the west lawn books you calculate the VCG and LCG based on the moments of all of the added weights to the hull (tanks, batteries, etc.) I just don’t see how this factors in the hull and having it trimmed correctly before you start factoring those things in.
     
  13. TANSL
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    TANSL Senior Member

    You need, in the first place, to finish defining your boat. You can not calculate the weight or estimate how the boat will float if you do not fully define it. That done, you should perform some calculation and I do not know if you are prepared or if you intend to do them. It seems not because, apparently, in 3 days you have not advanced anything, although you have been given some advice about it. If you advance a little more in the definition of the boat and if you show us the file with the 3D model perhaps someone could calculate something for you.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I think the deadrise angle is a little too shallow, you are going to have a boat that corners very flat, and slams crossing boat wakes etc. You could also find it bow steers crossing those wakes. 7 or 8 degrees at the transom would be better. Also, I don't think you need be pedantic about the LCG, this will be more affected by the non-fixed weights, and in planing hulls, there is usually generous margin for error, as underway they tend to trim in a small range, especially once at speed, and especially the flatter bottomed boats.
     
    Last edited: Nov 2, 2018

  15. TANSL
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    TANSL Senior Member

    If we forget the initial question (weights and cog) and we look at the shapes of the boat, we would probably have to change several things to improve its behavior. In addition, it would be necessary to know the boat's material and the planned construction system to introduce, probably, other improvements.
     
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